<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div style="position:absolute; background:blue" id="div">我是div</div>
    <script>

        var tween = {
        linear: function( t, b, c, d ){
            return c*t/d + b;
        },
        easeIn: function( t, b, c, d ){
            return c * ( t /= d ) * t + b;
        },
        strongEaseIn: function(t, b, c, d){
            return c * ( t /= d ) * t * t * t * t + b;
        },
        strongEaseOut: function(t, b, c, d){
            return c * ( ( t = t / d -1) * t * t * t * t + 1 ) + b;
        },
        sineaseIn: function( t, b, c, d ){
            return c * ( t /= d) * t * t + b;
        },
        sineaseOut: function(t, b, c, d){
            return c * ( ( t = t / d -1) * t * t + 1 ) + b;
        }
        };

        var Animate = function( dom ){
            this.dom = dom;                   // 进行运动的dom节点
            this.startTime = 0;               // 动画开始时间
            this.startPos = 0;                // 动画开始时，dom节点的位置，即dom的初始位置
            this.endPos = 0;                  // 动画结束时，dom节点的位置，即dom的目标位置
            this.propertyName = null;         // dom节点需要被改变的css属性名
            this.easing = null;               // 缓动算法
            this.duration = null;             // 动画持续时间
        };
        Animate.prototype.start = function( propertyName, endPos, duration, easing ){
            this.startTime = +new Date;        // 动画启动时间
            this.startPos = this.dom.getBoundingClientRect()[ propertyName ];  // dom节点初始位置
            this.propertyName = propertyName;  // dom节点需要被改变的CSS属性名
            this.endPos = endPos;  // dom节点目标位置
            this.duration = duration;   // 动画持续时间
            this.easing = tween[ easing ];  // 缓动算法

            var self = this;
            var timeId = setInterval(function(){      // 启动定时器，开始执行动画
              if ( self.step() === false ){           // 如果动画已结束，则清除定时器
                  clearInterval( timeId );
              }
          }, 19 );
        };

        Animate.prototype.step = function(){
            var t = +new Date;        // 取得当前时间
            if ( t >= this.startTime + this.duration ){       // (1)
              this.update( this.endPos );   // 更新小球的CSS属性值
              return false;
            }
            var pos = this.easing( t - this.startTime, this.startPos,
              this.endPos - this.startPos, this.duration );
            // pos为小球当前位置
            this.update( pos );    // 更新小球的CSS属性值
        };

        Animate.prototype.update = function( pos ){
            this.dom.style[ this.propertyName ] = pos + 'px';
        };

        var div = document.getElementById( 'div' );
        var animate = new Animate( div );

        // animate.start( 'left', 500, 1000, 'strongEaseOut' );
        animate.start( 'top', 1500, 500, 'strongEaseIn' );
    </script>
</body>
</html>